In the field of optical storage, it has been an important and well-developed technology to record digital signals of 0 and 1 generated by the change of refractive index of a phase-change material in different crystalline states, for example, an optical recording layer of an optical disk (CD, DVD). In 1986, investigators of International Business Machines Corporation (IBM) found that a phase-change alloy of germanium-stibium-tellurium (Ge—Sb—Te, GST) converts rapidly in phase by means of the transfer of electrical signals, making this phase-change material be extensively applied to a flash memory that can be operated to transfer electronic signals.
Along with the rapid growth of and increasing demand for information, it is desired to store information in a faster way and for a longer period. The memory feature of non-volatile phase-change memories mainly comes from a resistance change generated by a reversible transition of different phases between a non-conducting and a conducting state of the material. For example, an irregular semiconductor alloy of chalcogenide having Ge, Se and Te in an amorphous state can be transformed from an alloy in a chaotically amorphous state into a conductor in a well-crystallized state within dozens of nano-seconds by applying adequate electrical power or light power. Compared with conventional flash memories, such a phase-change memory has the advantages, such as shorter time for random access, higher processing ability for reading and writing, and more times of rewriting. In addition, the process thereof is simple and has high potential to produce a small-volume memory unit having high density and high capacity. It thus allows the phase-change material to be capable of meeting the oncoming 45-nanometer requirement for fabricating flash memories.
The phase-change memory utilizes the transition between thermal energy and potential energy to form two distinguishably stable states. In the past, the resistance in a memory unit of a phase-change memory (PRAM) was relatively low, for instance, just several thousand ohms, and the current of the unit had a high loss owing to the resistance of a transistor. In 2005 International Electron Devices Meeting (IEDM), Hitachi published a storage unit that is characterized by adding oxygen atoms in the GST film. After combing with germanium atoms, the added oxygen atoms disperse evenly in the GST film so that the film is in a state of small particle size and multi-crystal, thereby increasing the resistance value of the memory unit. The resistance is increased to 50 kΩ, so as to transform most energy of the current in the storage unit into thermal energy required for phase change, thereby increasing the phase-change efficiency and reducing the power consumption while writing. Such storage unit is fabricated by using 0.13-micrometer process technology, and the voltage/current required for data writing can be reduced to 1.5 V/100 μA, so as to meet the practical requirement of about 150 μW of phase-change power. However, the method of increasing the resistance by adding impurities, such as oxygen atoms and nitrogen atoms, in the GST film increases variation of characteristic due to a deviation of the addition amount of impurities in different units. Therefore, it is difficult to control the characteristics of the above storage unit.
Additionally, in 2006 International Electron Devices Meeting (IEDM), the technology of decreasing writing current without the addition of impurities was published. Such technology is to dispose a layer of Ta2O5 film having a heat-insulating effect between the GST film and the bottom electrode, so as to change the crystalline state of the GST film in low-current condition, and simultaneously, to increase the adhesion between the GST film and the bottom electrode by means of the Ta2O5 film.
However, with respect to the GST, the resetting process is subject to be interfered because of the material having two crystalline states, so that the resistance value of the reset amorphous state is not high enough and an incomplete reset may easily occur. Therefore, it is desired to have a phase-change material and a phase-change storage unit that can not only rapidly achieve the phase-change characteristic in low critical power condition, but also have a great discrepancy between high and low resistance states and avoid an incomplete reset.